1. Field of the Invention
This invention pertains to the application of cryogenic technology to life support systems. More particularly, the invention pertains to a dewar for storing and delivering cryogenic and a rapid fill process for the dewar.
2. Description of the Prior Art
"Cryogenic" is a term used to describe physical conditions where the temperature is less than approximately 123K. A "cryogenic fluid" may be defined as a fluid whose temperature is less than approximately 123K and which boils (i.e., changes state from liquid to gas) at temperatures less than approximately 110K (-262.degree. F., -163.degree. C.) at atmospheric pressure. A cryogenic fluid may therefore be either a gas or a liquid.
Examples of cryogenic fluids include both nitrogen and oxygen (the primary components of "liquid air") as well as hydrogen, helium and methane. In accord with well known laws of nature, cryogenic fluids may boil at lower temperatures when they are under lower pressures or at higher temperatures under higher pressures. The term "cryogen" as used herein shall refer to a cryogenic fluid and the term "cryogenic technology" shall refer to knowledge, techniques, and equipment for harnessing physical properties of cryogenic fluids to practical applications.
Cryogenic technology has been employed in a wide variety of diverse fields. The field of portable life support systems has seen a resurgence of interest in cryogenic technology. Many portable life support systems utilizing cryogenic fluids store a liquid cryogen in a vacuum insulated pressure vessel from which liquid cryogen is delivered to other parts of the life support system. Typically, the pressure vessel is jacketed by an insulative housing, the space between the pressure vessel and the insulative housing being evacuated and sometimes filled with multi-layered insulation or reflective powders. This type of insulated pressure vessel is typically called a "dewar".
Any dewar (or insulated pressure vessel) used in a portable life support system will contain gas and, if filled, liquid cryogen. With the exception of portable life support systems used in micro-gravity or zero-gravity environments, most portable life support systems use dewars which rely on the force of gravity to separate liquid cryogen from gaseous cryogen. This separation is advantageous because gaseous cryogen can be pressurized to provide a motive force in delivering liquid cryogen from the dewar and because it enables control over whether liquid or gas is delivered from the dewar. Portable life support system designers therefore take advantage of the natural properties of the cryogen to deliver liquid cryogen from the dewar by pressurizing the separated gaseous cryogen.
Some of the current efforts at portable life support system design have focused on the use of liquid cryogen as part of a cooling loop for the system user to regulate the user's body temperature. The heat exchange process in the cooling loop warms the liquid cryogen, generally converting the liquid cryogen to gaseous cryogen. If the liquid cryogen is "liquid air", the gaseous cryogen can be warmed to provide an air supply for the breathing loop of the system. It is consequently necessary in this type of portable life support system to provide a constant, uninterrupted flow of liquid cryogen from the dewar and the ability to rapidly refill the dewar when the level of liquid cryogen is low. By implication, it is also necessary to be able to determine the amount of liquid cryogen in the dewar with high certainty.
The drawback to relying on gravity for separation therefore is that the liquid cryogen will shift positions within the dewar whenever the orientation of the dewar is changed with respect to gravity. The dewar for a portable life support system is usually worn on the back of the system user, so whenever the user bends at the waist (as opposed to standing up straight) the orientation of the dewar with respect to gravity changes. This change can occur in one, or both, planes of movement: (1) forward and back, and (2) side to side.
The shift in position of the dewar's liquid contents can expose the port through which liquid cryogen is delivered from the dewar during such movements regardless of which plane the movement takes place. When the port becomes exposed, the pressurized gaseous cryogen escapes through the port. This depressurizes the dewar, thereby eliminating the motive force and interrupting the delivery of liquid cryogen. For instance, if someone wearing a portable life support system stoops or bends over as if to lift something, the port may become exposed thereby allowing pressurized gas to escape through the port and interrupting delivery of liquid cryogen until the port is once again immersed in the liquid cryogen and pressure is restored to the dewar.
Another problem with liquid cryogen dewars is that current filling procedures require that stores of liquid cryogen be kept on hand. Liquid cryogens are purchased and stored until such time as they are needed. Where the liquid cryogen of choice is liquid air, both liquid oxygen and liquid nitrogen of breathable quality must be kept on hand, mixed when needed, and then decanted (or allowed to flow in response to a pressure gradient) into the dewar.
These filling and mixing procedures, however, are laborious and time consuming because of system requirements and the physical properties of liquid air. The liquid air mix of liquid oxygen and liquid nitrogen must be carefully produced and maintained and the improper balance in the amounts of liquid oxygen and liquid nitrogen is very undesirable. Liquid air therefore cannot generally be stored because the liquid nitrogen component will boil off, thereby leaving the liquid air merely an oxygen enriched oxygen nitrogen mix.
The inability to store liquid air for longer than about a day causes many problems for some portable life support uses such as fire fighting. Since fires cannot generally be predicted, the liquid air must be mixed under emergency conditions. Complex procedures necessary for handling liquid oxygen and the requisite care in mixing to obtain the proper percentages of liquid air components require time and lead to the loss of valuable response time.
An alternative to keeping liquid cryogen on hand would be to produce liquid cryogen on-site and then fill the dewar. Current processes for liquefying a gas to a liquid cryogenic fluid are predicated on the manipulation of the pressure and temperature of the substance from a gas at supercritical temperatures and pressures to a liquid at subcritical temperatures and pressures. Every substance has a characteristic "critical temperature" which is defined as the highest temperature at which a distinct liquid phase of the substance exists. Every substance also has a characteristic "critical pressure" which is the pressure at or above which there is no distinction between the liquid and gaseous phases of the substance.
The critical temperatures and pressures for most common cryogens are known and, for nitrogen, oxygen, and air (an oxygen/nitrogen mixture emulating earth's atmosphere), are listed in Table 1.
TABLE 1 ______________________________________ CRITICAL CRYOGEN PRESSURES CRITICAL TEMPERATURES ______________________________________ NITROGEN 25 atm 126K (-233.degree. F., -147.degree. C.) OXYGEN 50 atm 155K (-182.degree. F., -118.degree. C.) AIR 38 atm 133K (-220.degree. F., -140.degree. C.) ______________________________________
Temperatures and pressures above the critical point values such as those listed in Table 1 are termed "supercritical" and temperatures and pressures below the critical point values are termed "subcritical". A cryogen may, depending on temperature and pressure, also be "supercritical" a term indicating that the cryogen is neither gas nor liquid but still exhibits physical properties of both.
Gas liquefaction processes generally begin with a gas at supercritical temperature and pressure, cool the gas to a subcritical temperature, and then pass the subcritically cooled substance through an expansion valve to produce a liquid at subcritical pressures and temperatures. However, known liquefaction processes are very time consuming because the objective is to produce as much liquid cryogen as possible with as little work as possible. A liquefaction process for a portable life support system must necessarily be different because (1) the chief objective is short fill time, and (2) the amount of work necessary to achieve liquefaction is not a governing factor. Thus, current fill processes are undesirable for portable life support systems.
Regardless of whether liquid air is produced on site or is mixed from stored liquid cryogens, it is desirable to refill the dewar as infrequently as possible to extend the activity time of the system user. This requires an accurate determination of liquid cryogen levels in the dewar at virtually all times. Current techniques employ a capacitance gauge in the dewar which distinguishes gas from liquid by their differing dielectrics. The capacitance of the gauge varies with the level of liquid, and so the shifting of liquid cryogen within the dewar caused by user movement also prohibits accurate determination of liquid cryogen levels in the dewar.
It is therefore an object of this invention to provide a dewar for the delivery of a liquid cryogenic fluid without interruption resulting from changes in orientation with regard to the field of gravity.
It is a further object of this invention to provide such a dewar for use in portable life support systems.
It is a still further object of this invention to provide such a dewar that can be filled more rapidly than conventional dewars without large liquid cryogen storage.
It is a still further object of this invention that it employs a new process for filling a dewar with liquid cryogen that is quicker than conventional techniques.
It is a still further object of this invention that it provides highly accurate indications of liquid cryogen levels in the dewar at virtually all times.